2 Anatomy CYTOMORPHOLOGY AND ULTRASTRUCTURE The description of the algal cell will proceed from the outside structures to the inside components. Details will be given only for those structures that are not comparable with analogue structures found in most animals and plants. The reader is referred to a general cell biology textbook for the structure not described in the following. OUTSIDE THE CELL Cell surface forms the border between the external word and the inside of the cell. It serves a number of basic functions, including species identification, uptake and excretion/secretion of various compounds, protection against desiccation, pathogens, and predators, cell signaling and cell–cell interaction. It serves as an osmotic barrier, preventing free flow of material, and as a selec- tive barrier for the specific transport of molecules. Algae, besides naked membranes more typical of animal cells and cell walls similar to those of higher plant cells, possess a wide variety of cell sur- faces. The terminology used to describe cell surface structures of algae is sometimes confusing; to avoid this confusion, or at least to reduce it, we will adopt a terminology mainly based on that of Presig et al. (1994). Cell surface structures can be grouped into four different basic types: . Simple cell membrane (Type 1) . Cell membrane with additional extracellular material (Type 2) . Cell membrane with additional intracellular material in vesicles (Type 3) . Cell membrane with additional intracellular and extracellular material (Type 4) Type 1: Simple Cell Membrane This cell surface consists of a simple or modified plasma membrane. The unit membrane is a lipid bilayer, 7–8 nm thick, rich of integral and peripheral proteins. Several domains exist in the mem- brane, each distinguished by its own molecular structure. Some domains have characteristic carbo- hydrate coat enveloping the unit membrane. The carbohydrate side chains of the membrane glycolipids and glycoproteins form the carbohydrate coat. Difference in thickness of plasma mem- brane may reflect differences in the distribution of phospholipids, glycolipids, and glycoproteins (Figure 2.1). A simple plasma membrane is present in the zoospores and gametes of Chlorophyceae, Xantho- phyceae (Heterokontophyta), and Phaeophyceae (Heterokontophyta), in the zoospores of the Eustigmatophyceae (Heterokontophyta), and in the spermatozoids of Bacillariophyceae (Hetero- kontophyta). This type of cell surface usually characterizes very short-lived stages and, in this transitory naked phase, the naked condition is usually rapidly lost once zoospores or gametes have ceased swimming and have become attached to the substrate, as wall formation rapidly ensues. A simple cell membrane covers the uninucleate cells that form the net-like plasmodium of the Chlorarachniophyta during all their life history. Most Chrysophyceae occur as naked cells, whose plasma membrane is in direct contact with water, but in Ochromonas, the membrane is covered with both a carbohydrate coat and surface blebs and vesicles, which may serve to trap bacteria and other particles that are subsequently engulfed as food. The properties of the membrane or its domains may change from one stage in the life cycle to the next. 35 © 2006 by Taylor & Francis Group, LLC Type 2: Cell Surface with Additional Extracellular Material Extracellular matrices occur in various forms and include mucilage and sheaths, scales, frustule, cell walls, loricas, and skeleta. The terminology used to describe this membrane-associated material is quite confusing, and unrelated structures such as the frustule of diatoms, the fused scaled cover- ing of some prasynophyceae, and the amphiesma of dinoflagellates have been given the same name, that is, theca. Our attempt has been to organize the matter in a less confusing way (at least in our opinion). Mucilages and Sheaths These are general terms for some sort of outer gelatinous covering present in both prokaryotic and eukaryotic algae. Mucilages are always present and we can observe a degree of development of a sheath that is associated with the type of the substrate the cells contact (Figure 2.2). All cyano- bacteria secrete a gelatinous material, which, in most species, tends to accumulate around the cells or trichome in the form of an envelope or sheath. Coccoid species are thus held together to form colonies; in some filamentous species, the sheath may function in a similar manner, as in the formation of Nostoc balls, or in development of the firm, gelatinous emispherical domes of the marine Phormidium crosbyanum. Most commonly, the sheath material in filamentous species forms a thick coating or tube through which motile trichomes move readily. Sheath production is a continuous process in cyanobacteria, and variation in this investment may reflect different physiologi- cal stages or levels of adaptation to the environment. Under some environmental conditions the sheath may become pigmented, although it is ordinarily colorless and transparent. Ferric hydroxide or other iron or metallic salts may accumulate in the sheath, as well as pigments originating within the cell. Only a few cyanobacterial exopolysaccharides have been defined structurally; the sheath of Nostoc commune contains cellulose-like glucan fibrils cross-linked with minor monosaccharides, and that of Mycrocystis flos-aquae consists mainly of galacturonic acid, with a composition similar to that of pectin. Cyanobacterial sheaths appear as a major component of soil crusts found throughout the world, from hot desert to polar regions, protecting soil from erosion, favoring water retention and nutrient bio-mobilization, and affecting chemical weathering of the environment they colonize. FIGURE 2.1 Schematic drawing of a simple cell membrane. 36 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC In eukaryotic algae, mucilages and sheaths are present in diverse divisions. The most common occurence of this extracellular material is in the algae palmelloid phases, in which non-motile cells are embedded in a thick, more or less stratified sheath of mucilage. This phase is so-called because it occurs in the genus Palmella (Chlorophyceae), but it occurs also in other members of the same class, such as Asterococcus sp., Hormotila sp., Spirogyra sp., and Gleocystis sp. A palmelloid phase is present also in Chroomonas sp. (Cryptophyceae) and in Gleodinium montanum vegetative cells (Dynophyceae) and in Euglena gracilis (Euglenophyceae) (Figure 2.3). Less common are the cases in which filaments are covered by continuous tubular layers of mucilages and sheath. It occurs in the filaments of Geminella sp. (Chlorophyceae). A more specific covering exists in the filaments of Phaeothamnion sp. (Chrysophyceae), because under certain growth conditions, cells of the fila- ments dissociate and produce a thick mucilage that surround them in a sort of colony resembling the palmelloid phase. Scales Scales can be defined as organic or inorganic surface structures of distinct size and shape. Scales can be distributed individually or arranged in a pattern sometimes forming an envelope around FIGURE 2.2 Transmission electron microscopy image of the apical cell of Leptolyngbya spp. trichome in longitudinal section. The arrows point to the mucilaginous sheath of this cyanobacterium. Inside the cell osmiophylic eyespot globules are present. (Bar: 0.15 mm). (Courtesy of Dr. Patrizia Albertano.) Anatomy 37 © 2006 by Taylor & Francis Group, LLC the cell. They occur only in eukaryotic algae, in the divisions of Heterokontophyta, Haptophyta, and Chlorophyta. They can be as large as the scales of Haptophyta (1 mm), but also as small as the scales of Prasynophyceae (Chlorophyta) (50 nm). There are at least three distinct types of scales: non-mineralized scales, made up entirely of organic matter, primarily polysaccharides, which are present in the Prasynophyceae (Chlorophyta); scales consisting of calcium carbonate crystallized onto an organic matrix, as the coccoliths produced by many Haptophyta; and scales constructed of silica deposited on a glycoprotein matrix, formed by some members of the Heterokontophyta. Most taxa of the Prasinophyceae (Chlorophyta) possess several scale types per cell, arranged in 1–5 layers on the surface of the cell body and flagella, those of each layer having a unique mor- phology for that taxon. These scales consist mainly of acidic polysaccharides involving unusual 2-keto sugar acids, with glycoproteins as minor components. Members of the order Pyramimona- dales exhibit one of the most complex scaly covering among the Prasinophyceae. It consists of three layers of scales. The innermost scales are small, square, or pentagonal; the intermediate scales are either naviculoid, spiderweb-shaped, or box shaped (Figure 2.4); the outer layer consists of large basket or crown-shaped scales. It is generally accepted that scales of the Prasinophyceae are syn- thesized within the Golgi apparatus; developing scales are transported through the Golgi apparatus by cisternal progression to the cell surface and released by exocytosis. In some Prasynophyceae genera such as Tetraselmis and Scherffelia, the cell body is covered entirely by fused scales. The scale composition consists mainly of acidic polysaccharides. These scales are produced only during cell division. They are formed in the Golgi apparatus and their development follow the route already described for the scales. After secretion, scales coalesce extracellularly inside the par- ental covering to form a new cell wall. In the Haptophyta, cells are typically covered with external scales of varying degree of com- plexity, which may be unmineralized or calcified. The unmineralized scales consist largely of complex carbohydrates, including pectin-like sulfated and carboxylated polysaccharides, and cellulose-like polymers. The structure of these scales varies from simple plates to elaborate, spectacular spines and protuberances, as in Chrysochromulina sp. (Figure 2.5) or to the unusual spherical or clavate knobs present in some species of Pavlova. Calcified scales termed coccoliths are produced by the coccolithophorids, a large group of species within the Haptophyta. In terms of ultrastructure and biomineralization processes, two FIGURE 2.3 Palmelloid phase of Euglena gracilis. (Bar: 10 mm.) 38 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC very different types of coccoliths are formed by these algae: heterococcoliths, (Figure 2.6) and holococcoliths (Figure 2.7). Some life cycles include both heterococcolith and holococcolith- producing forms. In addition, there are a few haptophytes that produce calcareous structures that do not appear to have either heterococcolith or holococcolith ultrastructure. These may be products of further biomineralization processes, and the general term nannolith is applied to them. Heterococcoliths are the most common coccolith type, which mainly consist of radial arrays of complex crystal units. The sequence of heterococcolith development has been described in detail in Pleurochrysis carterae, Emiliana huxleyi, and the non-motile heterococcolith phase of Coccolithus pelagicus. Despite the significant diversity in these observations, a clear overall pattern is discern- ible in all cases. The process commences with formation of a precursor organic scale inside Golgi- derived vesicles; calcification occurs within these vesicles with nucleation of a protococcolith ring FIGURE 2.5 Elaborate body scale of Chrysochromulina sp. FIGURE 2.4 Box shaped scales of the intermediate layer of Pyramimonas sp. cell body covering. Anatomy 39 © 2006 by Taylor & Francis Group, LLC of simple crystals around the rim of the precursor base-plate scale. This is followed by growth of these crystals in various directions to form complex crystal units. After completion of the coccolith, the vesicle dilates, its membrane fuses with the cell membrane and exocytosis occur. Outside the cell, the coccolith joins other coccoliths to form the coccosphere, that is the layer of coccoliths surrounding the cell (cf. Chapter 1, Figure 1.35). Holococcoliths consist of large numbers of minute morphologically simple crystals. Studies have been performed on two holococcolith-forming species, the motile holococcolith phase of Coccolithus pelagicus and Calyptrosphaera sphaeroidea. Similar to the heteroccoliths, the holococcoliths are FIGURE 2.7 Holococcolith of Syracosphaera oblonga. FIGURE 2.6 Heterococcolith of Discosphaera tubifera. 40 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC underlain by base-plate organic scales formed inside Golgi vesicles. However, holococcolith calcifi- cation is an extracellular process. Experimental evidences revealed that calcification occurs in a single highly regulated space outside the cell membrane, but directly above the stack of Golgi vesicles. This extracellular compartment is covered by a delicate organic envelope or “skin.” The cell secretes calcite that fills the space between the skin and the base-plate scales. The coccosphere grows pro- gressively outward from this position. As a consequence of the different biomineralization strategies, heterococcoliths are more robust than the smaller and more delicate holococcoliths. Coccolithophorids, together with corals and foraminifera, are responsible for the bulk of oceanic calcification. Their role in the formation of marine sediment and the impact their blooms may exert on climate change will be discussed in Chapter 4. Members of the Chrysophyceae (Heterokontophyta) such as Synura sp. and Mallomonas sp. are covered by armor of silica scales, with a very complicated structure. Synura scales consists of a perforated basal plate provided with ribs, spines, and other ornamentation (Figure 2.8). In Mallomonas, scales may bear long, complicated bristles (Figure 2.9). Several scale types are pro- duced in the same cell and deposited on the surface in a definite sequence, following an imbricate, often screw-like pattern. Silica scales are produced internally in deposition vesicles formed by the chrysoplast endoplasmic reticulum, which function as moulds for the scales. Golgi body vesicles transporting material fuse with the scale-producing vesicles. Once formed the scale is extruded from the cell and brought into correct position on the cell surface. Frustule This structure is present only in the Bacillariophyceae (Heterokontophyta). The frustule is an ornate cell membrane made of amorphous hydrated silica, which displays intricate patterns and designs unique to each species. This silicified envelope consists of two overlapping valves, an epitheca and a slightly smaller hypotheca. Each theca comprises a highly patterned valve and one or more girdle bands (cingula) that extend around the circumference of the cell, forming the region of theca overlay. Extracellular organic coats envelop the plasma membrane under the siliceous FIGURE 2.8 Ornamented body scale of Synura petersenii. Anatomy 41 © 2006 by Taylor & Francis Group, LLC frustule. They exist in the form of both thick mucilaginous capsules and thin tightly bound organic sheaths. The formation of the frustule has place in the silica deposition vesicles, derived from the Golgi apparatus, wherein the silica is deposited. The vesicles eventually secrete their finished product onto the cell surface in a precise position. Diatoms can be divided artificially in centric and pennate because of the symmetry of their frus- tule. In centric diatoms, the symmetry is radial, that is, the structure of the valve is arranged in refer- ence to a central point (Figure 2.10). However, within the centric series, there are also oval, triradiate, quadrate, and pentagonal variation of this symmetry, with a valve arranged in reference to two, three, or more points. Pennate diatoms are bilaterally symmetrical about two axes, apical and trans-apical, or only in one axis, (Figure 2.11); some genera possess rotational symmetry, (cf. Chapter 1, Figure 1.30). Valves of some pennate diatoms are characterized by an elongated fissure, the raphe, which can be placed centrally, or run along one of the edges. At each end of the raphe and at its center there are thickenings called polar and central nodules. Addiction details in the morphology of the frustule are the stria, lines composed of areolae, and pores through the valve that can go straight through the structure, or can be constricted at one side. Striae can be separated by thickened areas called costae. Areolae are passageways for the gases, nutrients exchanges, and mucilage secretion for movement and attachment to substrates or other cells of colony. Other pores, also known as portules, are present on the surface of the valve. FIGURE 2.9 Body scale of Mallomonas crassisquama. 42 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC There are two types of portules: fultoportulae (Figure 2.12) found only in the order Thalassiosirales and rimoportulae (Figure 2.13), which are universal. The structure of the fultoportulae is an external opening on the surface of the valve extended or not into a protruding structure (Figure 2.12). The other end penetrates the silica matrix and is supported with two to five satellite pores. The portules FIGURE 2.10 Triceratium sp., a centric diatom. FIGURE 2.11 Rhoicosphenia sp., a pennate diatom. Anatomy 43 © 2006 by Taylor & Francis Group, LLC function in the excretion of several materials, among them are b-chitin fibrils. These fibrils are man- ufactured in the conical invaginations in the matrix, under the portule. This may be the anchoring site for the protoplast. The rimoportula is similar to the fultoportulae, except that it has a simpler inner structure. The rimoportula does not have satellite pores in the inner matrix. However, the rimoportula does have some elaborate outer structures that bend, have slits, or are capped. Some- times the valve can outgrow beyond its margin in structures called setae that help link adjacent cells into linear colonies as in Chaetoceros spp., or possess protuberances as in Biddulphia spp. that allow the cells to gather in zig-zag chains (Figure 2.14). In other genera such as Skeletonema the valve presents a marginal ridge along its periphery consisting of long, straight spines, which make contact between adjacent cells, and unite them into filaments. Some genera also possess a labiate process, a tube through the valve with internally thickened sides that may be flat or elevated. Diatoms are by far the most significant producer of biogenic silica, dominating the marine silicon cycle. It is estimated that over 30 million km 2 of ocean floor are covered with sedimentary deposits of diatom frustules. The geological and economical importance of these silica coverings as well as the mechanism of silica deposition will be discussed in Chapter 4. Cell Wall A cell wall, defined as a rigid, homogeneous and often multilayered structure, is present in both prokaryotic and eukaryotic algae. In the Cyanophyta the cell wall lies between the plasma membrane and the mucilaginous sheath; the fine structure of the cell wall is of Gram-negative type. The innermost layer, the electron-opaque layer or peptidoglycan layer, overlays the plasma membrane, and in most cyano- bacteria its width varies between 1 and 10 nm, but can reach 200 nm in some Oscillatoria species. Regularly arranged discontinuities are present in the peptidoglycan layer of many cyanobacteria; pores are located in single rows on either side of every cross wall, and are also uniformly distributed over the cell surface. The outer membrane of the cell wall appears as a double track structure tightly connected with the peptidoglycan layer; this membrane exhibits a number of evaginations repre- senting sites of extrusion of material from the cytoplasm through the wall into the slime. The cell wall of Prochlorophyta is comparable to that of the cyanobacteria in structure and contains muramic acid. Eukaryotic algal cell wall is always formed outside the plasmalemma, and is in many respects comparable to that of higher plants. It is present in the Rhodophyta, Eustigmatophyceae (Figure 2.15a and 2.15b), Phaeophyceae (Heterokontophyta), Xanthophyceae (Heterokontophyta), FIGURE 2.12 Fultoportula of Thalassiosira sp. FIGURE 2.13 Rimoportula of Stephanodiscus sp. 44 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC [...]... lorica is calcified, ornamented, and is composed FIGURE 2.16 Tree-like arrangement of Dinobryon sp cells showing their loricas © 2006 by Taylor & Francis Group, LLC 48 Algae: Anatomy, Biochemistry, and Biotechnology of two cup-shaped parts that separate at reproduction In Pteromonas, the lorica extend into a projecting wing around the cell and is composed of two shell-like portions joined at the wings... zone and is located latero-ventrally with respect to the axoneme and the cell body In cross-thin sections of isolated and demembranated flagella, the rod appears hollow with an outer diameter of 90 nm Images obtained from negative staining preparations show that the rod is made up of several coiled filaments, with a diameter of 22 nm, forming a seven-start left-handed helix with a pitch of 458 and a... 2006 by Taylor & Francis Group, LLC 58 Algae: Anatomy, Biochemistry, and Biotechnology algae such as Chlorophyceae and Charopyceae Within this group, there are few genera whose flagella differ in length, which are termed “anisokont.” Description of flagella anatomy will proceed from outside to the inside, from the surface features and components to the axoneme and additional inclusions to the structures... Heterokontophyta), and in members of the Euglenophyta and Dinophyta PFRs are complex and highly organized lattice-like structures that run parallel to the axoneme Among the different groups, PFRs are very similar structurally and biochemically In the Pedinellales, the membrane of the single emergent flagellum is expanded into a sheath or fin supported along the edge by the rod, which is cross-banded Owing to... sulcal plates, five postcingular plates, and two antapical plates © 2006 by Taylor & Francis Group, LLC 50 Algae: Anatomy, Biochemistry, and Biotechnology FIGURE 2.18 Diagram of the eight distinct categories of the dinoflagellate amphiesma Type 4: Cell Surface with Additional Extracellular and Intracellular Material Both the surface structure of the Cryptophyta and that of the Euglenophyta can be grouped... variable periodicities and thickness only during its contraction, but not in its relaxed or fully © 2006 by Taylor & Francis Group, LLC 66 Algae: Anatomy, Biochemistry, and Biotechnology FIGURE 2.37 Transmission electron microscopy image of the locomotory flagellum of E gracilis in longitudinal section showing the PFR (a) Schematic drawing of the PFR showing the coiled filaments and goblet-like projections... Taylor & Francis Group, LLC 70 Algae: Anatomy, Biochemistry, and Biotechnology FIGURE 2.42 Type 3 transition zone of Dinophyta (a), Glaucophyta (b), and Haptophyta (c) the number of gyres, which in a short flagellum may be as low as one This helix is present in Chrysophyceae, Xanthophyceae, and Eustigmatophyceae (Heterokontophyta) Type 5 (Figure 2.45) is characterized by the so-called “stellate pattern”;... most likely descendant of biflagellated cells Intermediate cases exist, which carry a short © 2006 by Taylor & Francis Group, LLC 56 Algae: Anatomy, Biochemistry, and Biotechnology FIGURE 2.25 Deep-etching image of E gracilis showing the mucus coating of the cell surface and the protoplasmic fracture of the cell membrane (Bar: 0.10 mm.) (Courtesy of Dr Pietro Lupetti.) second flagellum, as in Mantoniella... Group, LLC Algae: Anatomy, Biochemistry, and Biotechnology 52 FIGURE 2.20 Periplast of Chroomonas sp Euglenophyta possess an unusual membrane complex called the pellicle, consisting of the plasma membrane overlying an electron-opaque semicontinuous proteic layer made up of overlapping strips These strips or striae that can be described as long ribbons that usually arise in the flagellar pocket and extend... trailing flagellum of Ochromonas danica in both longitudinal (a) and transverse sections (b), showing the tripartite hairs (Bar: 0.25 mm.) © 2006 by Taylor & Francis Group, LLC 62 Algae: Anatomy, Biochemistry, and Biotechnology consist of a sheath about 240 – 300 nm in length, which represent the basic unit The units, each formed by loops, side arms and filaments, lie parallel to each other in the longitudinal . retention and nutrient bio-mobilization, and affecting chemical weathering of the environment they colonize. FIGURE 2.1 Schematic drawing of a simple cell membrane. 36 Algae: Anatomy, Biochemistry, and. tubifera. 40 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC underlain by base-plate organic scales formed inside Golgi vesicles. However, holococcolith calci - cation. sp. 44 Algae: Anatomy, Biochemistry, and Biotechnology © 2006 by Taylor & Francis Group, LLC Chlorophyceae, and Charophyceae (Chlorophyta). Generally, cell walls are made up of two com- ponents,